Hi Eli,
I'm not an expert, but from researching, my understanding was that travel insurance will cover expenses previously paid. I believe the only issue related to timing of when you buy the insurance is related to a waiting period for pre-existing health conditions. We made our first purchase relating to our trip last October and bought travel insurance in February for a June trip.
We ended up going with AIG's Travel Guard. I think it was their silver level, also known as Protect Assist. I thought this was the best reasonable combination. The amount we decided on will cover all of our prepaid expenses plus a little bit of expenses we will incur over there. For example, most of our hotels have a policy that you can cancel up a week, 2 days before, or the day of. We didn't include most of these costs as it would have greatly added to the cost of insurance. I tried to find a balance- but go with what makes you comfortable.